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This episode features the following technical highlights:
• National Drilling Initiative Delamerian Project – Sampling Workshop; and
• Discreet Impact Experiments Provide an Understanding of Percussive Fragmentation and Lead Strategies to Optimise Drilling.
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The MinEx CRC National Drilling Initiative Delamerian sampling workshop provided an overview of the field data acquisition and results for the Delamerian NDI. Researchers from Project 7.5 were invited to establish projects to analyse and interpret the new NDI samples.
Delamerian NDI project has provided a suite of geological samples through the Cenozoic Murray Basin and into the Devonian to Cambrian basement rocks in eastern South Australia. The sampling workshop brought together GSSA and researchers from academia and other government departments, aimed to add value to the next phase of the Delamerian NDI program and bring insights to the data collected during the drilling campaign.
The two day sampling workshop focused on summarising knowledge gained for different aspects of our work program, from sample preparation strategies, to geochemical and geochronological data, Hylogger as well as insights on geology over the core/chips. The workshop included core and sample viewing, contextual geology talks, outlines of GSSA work to date, planned research activities, and opportunities for MinEx 7.5 projects. Opportunities for research include:
• Ar geochronology for dating alteration systems
• Rb/Sr geochronology to tackle the age on fine grained volcanic rocks (hard with the standard techniques)
• Heat flow and petrophysical properties to help refine rock properties and models
• Pyrite/zircon chem for characterising fluids
• Age of the post-Delamerian sedimentary basin
• Using Low Temperature thermochronology such as Ar-Ar to date alteration and timing of uplift and exhumation
• Ettrick formation chemistry and impact of sea water chemistry on elevated base metals at the transition between Murray Group and Renmark Group
• Tracking the sulphides in the back -arc rocks of the Delamerian, and the porphyry-style deposits in the Adelaide Rift Complex
The Delamerian NDI sampling workshop has enabled GSSA to connect with Project 7.5 researchers and follow up opportunities to expand our knowledge of the formation of the Delamerian rocks undercover, and to track potential for economic mineralisation under deep sedimentary basins through the innovative drilling rig technology and successful deployment of core sampling by the CT rig.
Today we would like to introduce our Woody – or woody woodpecker – that is our discreet impact drilling equipment, designed to gain better understanding of the processes that control performance in percussion drilling.
In project 1, we aim at optimising drilling performance, or more precisely minimise the time on-bottom required to complete a borehole. And eventually, we aim at coupling optimisation with automation. Efficient adaptative control requires an efficient model of the system, meaning an understanding of how control parameters affect the drilling response, and how change in rock properties or bit wear affect the response.
In percussion drilling, there is some lab and field evidence of the existence of a sweet spot, or an optimal thrust – or weight-on-bit – applied on the drill bit that yields maximum rate of penetration and beyond which performance drops. In fact, during the first field trial of our monitoring equipment in the Pilbara, we confirmed the existence of sweet-spot and showed that the drillers most often drilled with excessive weight-on-bit with rate of penetration about a third of the optimal value.
There is however no consensus on the root cause (or causes) for this ideal drilling condition.
Does this sweet spot stem from the intrinsic nature of the bit-rock interaction in other words if I press an indenter against a rock surface with a static force and apply on top of it a given impact force, does the penetration exhibit a peak value for a given “optimal static force”?
Or is the sweet spot a by-product of the overall system dynamics which include not only the bit and the rock but also the hammer itself. In other words, is the sweet spot in part controlled by what is happening between impacts?
Gaining a better understanding of the mechanism that govern the sweet spot is essential to define the strategy to optimise drilling performance via control parameters but also via bit and hammer design.
This is where Woody comes into the picture. It is a quite unique, novel experimental setup, that is designed to drill with a standard percussion bit by successive – discreet impacts (one impact at a time) but without the overall hammer dynamics. Woody allows us to control the static weight-on-bit, but also the piston impact velocity and to increment the angular position between the bit and the rock between each impact, which allows us to drill forward one impact at a time.
There has been in the literature experimental work dedicated to the study of discreet percussive impact but without the ability to apply or vary weight-on-bit, nor the ability to apply successive impacts, with strong consequence on the robustness-repeatability of the results as they tended to consider just one isolated event often with the bit against a flat rock surface.
Woody has a range of quite unique measurements with uncommon sensors. Bit penetration during an impact occurs over about 1 ms, we therefore record at very high sampling rate (100’s of kHZ) bit penetration with special laser, we also record the piston velocity right before impact and the rebound velocity with high-speed proximity sensors located along the hammer barrel. But we also measure the elastic wave propagating through the bit shaft at GHz, which allows to track the energy transferred to the rock and reflected to the bit.
We have added many automation features to the equipment that allows us to perform 4-5 impacts per minute. Note that in order to initiate and propagate a borehole over sufficient distance to yield a reliable and robust measure of the average penetration per impact, we need to carry out about 150 to 200 impacts.
We have already carried about 3000 impacts with remarkable results with no equivalence to our knowledge in the literature. With these experiments not only, we aim to unlock one of the mystery of percussion drilling but also outline the strategy to optimise drilling performance in percussion drilling.